The present invention relates to noise reduction systems for television and more particularly relates to a noise reduction system for television which processes both vertical and horizontal information.
Unidimensional coring circuits have been known and used in noise reduction systems for television for many years. A common approach taken was to pass a video signal through a frequency-selective filter to separate high and low frequency components thereof with the high frequency components then containing the noise to be reduced. Then the high frequency component signal was passed through a symmetrical threshold circuit commonly referred to as a coring circuit. The coring circuit removed the center of the signal symmetrically without affecting amplitude excursions beyond the threshold level which defined the center or the "core." Then the cored, high frequency component signal was added to the low frequency component in an adding circuit to provide a restored, noise reduced video signal. That prior art approach was exemplified by U.S. Pat. No. 3,117,278 to Johnson (see FIG. 2 thereof) and also by U.S. Pat. No. 3,715,477 to Olson (see FIG. 1 thereof). Those coring circuits basically functioned as low pass filters at low amplitude levels only. Thus, those circuits resulted in loss of picture detail at low amplitude levels. For high amplitude signal levels, those prior art circuits had little, if any, effect. One drawback of those circuits which is overcome by the present invention was the relatively high threshold level required for effective noise reduction. That high threshold level necessarily removed from the picture some of the low level picture information.
One prior art solution to the drawback of loss of picture detail at low amplitude levels was preemphasis of the video signal at one end of a circuit path such as the transmission channel or tape recorder and corresponding deemphasis at the other end thereof. That approach was taken in U.S. Pat. No. 3,286,024 to Melchior and is a philosophy generally followed by Dolby in his U.S. Pats. Nos. 3,846,719, 3,828,280 and 3,903,485. Such an approach was also taken in the field of geophysical survey in the U.S. Pat. No. 2,817,715 to Blake.
One prior art system shown in U.S. Pat. No. 3,769,611 to Scaggs divided wide bandwidth signals such as a television signal up into multiple frequency components and separately processed each component. Although the processing of each of the components with the Scaggs system was smooth, the inherent problem of an unduly high threshold level with corresponding signal degradation remained while another practical problem of matching delays and amplitudes of different channels was introduced.
It has been more recently proposed to improve vertical aperture correction in television cameras by considering picture information bidimensionally in adjacent scan lines of a video signal. That approach was suggested in the Longuet U.S. Pat. No. 3,814,847. While the Longuet patent disclosed a bidimensional noise reduction system, the apparatus described therein for accomplishing noise reduction was hampered by a number of drawbacks overcome by the present invention. A principal drawback of the Longuet system was the provision of but a single coring or threshold circuit for both single horizontal line transitions and multiline vertical information transitions. That single-coring-circuit approach required a higher threshold level which removed more low level picture information than if the threshold were lowered. Thus, Longuet lost the noise reduction benefit of having a bidimensional device that responded differently to line transitions and to multiple-line vertical information transistions. The inherent drawback of the common coring circuit was that the threshold level had to be set at the highest noise level in both the horizontal and vertical domains.
Another drawback of the Longuet system was that it did not take into account the presence of encoded chroma information in the video channel (which is understandable in that the Longuet system was strictly for use at the camera). Thus, the Longuet system was limited to processing red, green, blue, and/or luminance channels at, e.g., the camera, prior to encoding. The present invention overcomes these and other limitations of the Longuet system.